For example, a state-of-the art system for assisting driving, operation, and maintenance is described in the June 1990 issue of the "Revue Generale des Chemins de Fer" ["General Railways Journal"].
In that journal, articles entitled "SACEM: objectifs et specifications" ["SACEM: aims and specifications"] pages 13 to 18, "Principes et fonctionnement du Systeme d'Aide a 1a Conduite, a 1'Exploitation, et a 1a Maintenance (SACEM)" ["Principles and workings of the System for Assisting Driving, Operation, and Maintenance (SACEM)"] pages 23 to 28, and "L'installation du systeme SACEM sur la ligne A du RER" ["Installing the SACEM system on RER line A"] pages 47 to 51, supply a detailed description of the system.
The "SACEM" system for assisting driving, operation, and maintenance is a traffic monitoring system designed for high-throughput rail transport systems.
The on-board equipment is composed of a computer associated with antennas. The antennas receive the continuous-transmission electrical signals (flowing through the rails) which supply a description of a portion of line to the trains. The antennas also make it possible to read the contents of messages transmitted by beacons at various locations.
The beacons employed by the system for assisting driving, operation, and maintenance are used to supply a precise geographical position marker to the train in the track description in its possession.
Three categories of beacon are currently employed to perform that function.
The first category may be referred to as a "running-initialization" beacon. That beacon supplies the information required for the train to locate itself for the first time. Until then, the train is not initialized.
The second category of beacon may be referred to as a "relocation beacon" and it is designed to provide a new setting for the measurement of the displacement of the train periodically (about every 500 meters).
The third category of beacon supplies information to the train locating a point at which the train leaves a zone monitored by the system for assisting driving, operation, and maintenance.
Because of their structure, those three categories of beacons can be read only when the train is moving.
Transmission is not hindered by the presence of snow, ice, water, or even ore or iron filings on the beacons.
The above-described speed-monitoring system includes beacons at various locations, i.e. passive ground beacons, enabling a reference in space to be obtained.
Each initialization beacon defines a stationary-initialization zone. On entering one of such monitoring zones, an initialization beacon is read while the train is moving. It is important to note that the initialization is performed while the train is running.
To enable initialization to be performed while the train is stationary, and therefore to enable the train to be monitored as soon as its on-board equipment is switched on, it must be possible to transmit the train-location information while the train is stationary. To be entirely safe, such transmission must be performed by continuous transmission, and must enable the train to locate itself in the track description supplied to it.